The present invention relates to improved optical amplifiers and lasers having both broad band and wide range specific band capability, and more particularly to an optical amplifier and laser based on solids containing semiconductor nanocrystals.
Optical amplifiers utilize a gain medium to amplify optical radiation. In an amplifier, a source excites the gain medium to produce a population inversion between high and low energy states of the gain medium. The excited gain medium can amplify optical radiation at energies overlapping the energy differences between the high and low energy states of the population inversion because stimulated emission of radiation from the medium is more efficient than absorption of light. In general, a laser utilizes a cavity to supply feedback to an excited gain medium to cause amplified spontaneous emission. A laser cavity can include a series of optical components, such as mirrors, arranged relative to the gain medium to reflect radiation back into the cavity and thereby provide feedback. For example, a gain medium can be placed into a stable or unstable resonator. Alternatively, amplified spontaneous emission can occur in an excited gain medium without external optical components if the gain medium has a length, L, and gain coefficient, G (cmxe2x88x921) sufficient to satisfy the expression:
G L greater than  greater than 1
where the gain coefficient, G, is related to the stimulated emission cross section and the difference in the population densities of the high and low energy states generated by the population inversion.
Conventional solid-state and gas lasers and amplifiers generally provide very specific spectral outputs depending upon the laser material. If a spectral output other than that achievable with available gain materials or a less specific spectral output is desired, dye lasers or tunable optical parametric oscillators (OPO) or amplifiers (OPA) can be used. Dye lasers are large and bulky and also require fluid components that can be toxic.
In general, a gain media includes a plurality of semiconductor nanocrystals. The gain medium can be used to amplify optical radiation or produce optical radiation by lasing. In particular, the gain medium includes concentrated solids of semiconductor nanocrystals, such as close-packed films of semiconductor nanocrystals, that provide high gain to produce optical amplification or lasing over short amplifier or cavity lengths.
In one aspect, a gain medium includes a concentrated solid having a plurality of semiconductor nanocrystals.
In another aspect, a laser includes an optical gain medium and a cavity arranged relative to the optical gain medium to provide feedback. The optical gain medium includes a concentrated solid having a plurality of semiconductor nanocrystals. The cavity can be a microcavity. For example, the cavity can have a length that is less than 1 mm.
In another aspect, a method of amplifying an optical signal includes directing an optical beam into a concentrated solid including a plurality of semiconductor nanocrystals.
In another aspect, a method of forming a laser includes arranging a cavity relative to an optical gain medium to provide feedback to the optical gain medium. The optical gain medium includes a concentrated solid including a semiconductor plurality of nanocrystals.
The concentrated solid can be substantially free of defects. A concentrated solid including defects produces loses, such as scatter, such that the solid does not provide gain to optical radiation. The solid can provide gain to an optical signal at an energy equal to or less than the maximum band gap emission of the nanocrystals. The solid also is capable of providing gain at energies in which a concentrated solid is substantially free of absorption.
Concentrated solids include greater than 0.2%, greater than 5%, greater than 10%, or greater than 15% by volume semiconductor nanocrystals. The each of the plurality of semiconductor nanocrystals includes a same or different first semiconductor material. The first semiconductor material can be a Group II-VI compound, a Group II-V compound, a Group III-VI compound, a Group III-V compound, a Group IV-VI compound, a Group I-III-VI compound, a Group II-IV-VI compound, or a Group II-IV-V compound, such as, for example, ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe, AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, GaSe, InN, InP, InAs, InSb, TlN, TlP, TlAs, TlSb, PbS, PbSe, PbTe, or mixtures thereof. Each first semiconductor material is overcoated with a second semiconductor material, such as ZnO, ZnS, ZnSe, ZnTe, CdO, CdS, CdSe, CdTe, MgO, MgS, MgSe, MgTe, HgO, HgS, HgSe, HgTe, AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb, TlN, TlP, TlAs, TlSb, TlSb, PbS, PbSe, PbTe, or mixtures thereof. Each first semiconductor material has a first band gap and each second semiconductor material has a second band gap that is larger than the first band gap. Each nanocrystal can have a diameter of less than about 10 nanometers. The plurality of nanocrystals have a monodisperse distribution of sizes. The plurality of nanocrystals have a plurality of monodisperse distribution of sizes. The plurality of monodisperse distribution of sizes can provide gain over a broad range of energies or over a plurality of narrow ranges, e.g., a FWHM of gain less than 75 nm, in which the gain maxima occur at a separate energy such that at least some of the narrow ranges do not overlap in energy. The concentrated solid of nanocrystals is disposed on a substrate such as glass. The concentrated solid of nanocrystals has a thickness greater than about 0.2 microns.
Optical gain from gain media including semiconductor nanocrystals occurs at excitation densities in excess of only one electron-hole (e-h) pair per nanocrystal. The semiconductor nanocrystals provide wide-range of tunable optical properties, e.g., emission and absorption wavelengths. The nanocrystals exhibit discrete atomic-like optical transitions in linear absorption and size dependent photoluminescence (PL), e.g., emission, spectra. The photoluminescence properties are tuned by changing the size of the semiconductor nanocrystal to change the semiconductor energy-gap. The semiconductor energy-gap increases, from a bulk semiconductor value, inversely with the radius (R) of the nanocrystal according to 1/R2. For example, the band gap in CdSe quantum dots can be changed from 1.7 eV (deep red) to 3.2 eV (ultraviolet) by reducing the dot radius from 10 nanometers (nm) to 0.7 nm. The semiconductor nanocrystals are synthesized with R between about 1 nm to 15 nm, e.g., less than 10 nm, with less than about 5 percent size dispersion. Semiconductor nanocrystals having a modified surface (e.g., by overcoating the nanocrystal with a shell of a wide-gap semiconductor) allow significant suppression of carrier surface trapping, to the enhance the room-temperature PL quantum efficiency up to 50 percent and higher. Gain media including semiconductor nanocrystals are produced quickly and at low cost. The semiconductor nanocrystals can be used in optical waveguide materials for optically interconnecting integrated circuits. Without needing optical and electro-optical tuning and filtering systems, gain media including different semiconductor nanocrystals, different sizes of semiconductor nanocrystals, or both, can provide optical gain at very broad bandwidth (i.e., a broad band of radiation energies), narrow bandwidth (i.e., a narrow band of radiation energies), or a plurality of narrow or broad bands at different radiation energies (i.e., the bands do not necessarily overlap in energy).
Other features, objects, and advantages will be apparent from the description and drawings, and from the claims.